1. Field of the Invention
The present invention generally relates to focused ion beam apparatuses, and more particularly to methods and devices for correcting beam irradiation positions in focused ion beam apparatuses, which overcomes the problems associated with a fine processing that employs ion beam irradiation in which an irradiated position of ion beam may shift during the fine processing.
2. Related Background Art
When a fine processing is conducted through irradiation of an ion beam, the irradiated position (or focused position) of the ion beam may shift due to a variety of factors, which may often make it difficult to conduct a specified fine processing. In particular, due to further miniaturization in the semiconductor device design rule, the positional accuracy that is required in a fine processing has reached a level of 100 nm or below. In order to realize such a positional accuracy, the amount of drift, i.e., a shift of irradiation position, during a fine processing needs to be restricted to about one length ({fraction (1/10)}) of the positional accuracy or less, which is about 10 nm or less.
In order to solve the problem of drifts of irradiation position during fine processings, improvements in terms of hardware have been made to reduce the amount of drift. However, the improvement in hardware has its limitation. Accordingly, methods that involve detection of the amount of drift to correct the beam irradiation position are commonly practiced.
The methods generally employ pattern matching techniques with image processing, by which the amount of drift can be detected through pattern matching of reference regions. However, in the fine processing that uses a focused ion beam, a series of fine proceedings are conducted through changing beam currents to be irradiated on a sample (which may be referred hereafter to as the “sample irradiation beam currents) for a rough processing, an intermediate processing and a finish processing. In order to change the sample irradiation beam currents, the aperture diameter of a variable aperture stop, which is one of components of the focused ion lens system, may be changed, or the ion lens condition may be changed. However, the changes in the conditions would change the position of sample irradiation beam. Accordingly, an image of the entire sample needs to be taken again for such a change in the focused lens system, and the beam irradiation position needs to be reset.
Also, when the sample irradiation beam current is increased, the beam diameter becomes larger, and an image obtained becomes more obscure compared to an image obtained when the sample irradiation beam current is small. Further, depending on samples, when the sample irradiation beam current is increased, a localized charge-up phenomenon occurs such that the brightness/contrast may also change. Accordingly, when a series of fine processings are performed through changing the sample irradiation beam current, it is difficult to correct drifts by the pattern matching technique. To overcome this difficulty, there has been proposed a pattern correction method in which reference patterns in dots are formed, and positions of the dots are referred to during the proceedings, and irradiation positions to be processed are corrected.
However, the conventional pattern correction method entails the following problems in forming the reference patterns in dots. One of the problems is that samples may not often have a flat surface on which dot patterns are formed. In this case, it is difficult to discriminate roughness (i.e., concave sections and convex sections) that are originally present on the sample surface from the dots formed. Accordingly, prior to forming dotted patterns on the surface of the sample, a deposition film needs to be formed by flowing a specified gas in areas adjacent to the dot patterns to be formed and irradiating a beam on the sample, and then the dot patterns need to be formed on the deposition film. The deposition film can cover up the roughness on the surface of the sample that is originally present on the sample.
While processing a specified region, the processing work may be stopped occasionally, and a reference pattern region where the dot patterns are formed is scanned and the amount of drift is measured. However, after repeatedly scanning the reference pattern region, the deposition film is finally etched, and the original surface roughness of the sample may possibly reveal. In order to prevent this incident, a thick deposition film needs to be formed. For example, when it takes 20 seconds to form a dot pattern, it would take 2 minutes to form a deposition film prior to forming the dot pattern.
Also, in measuring the amount of drift through recognizing the dot patterns, locations of the dots are obtained by image processing. However, when the dots are small, only a few of the dots have brightness recognizably different from that of the surrounding, in other words, there are only a few specific points. In measuring positions of the dots, the required measurement accuracy needs to be high enough to recognize a size smaller than a pixel size of an image that is obtained. However, when there are a few specific points with brightness recognizably different from that of the surrounding, the accuracy in measuring positions cannot be increased to a required level. If the size of each dot is enlarged, the measurement accuracy may improve as the number of specific points in a specified group increases. However, when the reference pattern region is scanned to obtain a reference pattern image, the entire group of the specific points may not possibly be contained inside the image area obtained by scanning the reference pattern region if the amount of drift is large. The amount of drift cannot be accurately measured without recognizing all of the specific points in the group. Otherwise, the accuracy in measuring positions may have to be sacrificed to a degree, and drift allowances may be increased if these specific points are to be used in measuring the amount of drift. The balance between the accuracy in measuring positions and the drift allowances must be taken into consideration to realize an appropriate drift correction state. For this reason, even when the creation of dot patterns is automated so that an operator can preset processing conditions, and dot patterns for drift correction are automatically created according to the preset processing conditions, it is difficult to guarantee if the drift correction operation is reliably performed.
When performing a cross-sectional processing or a transmission electron microscope (TEM) sample processing, a plurality of steps including a rough processing step, an intermediate processing step and a finish processing step are conducted. If the sample irradiation current of the focused ion beam is increased in order to increase the processing speed, the beam diameter of the focused ion beam becomes relatively large. In this case, a cross section that is formed by the processing becomes to have a slanted surface, in other words, does not become to be vertical (i.e., perpendicular) with respect to the plane surface of the sample. To obtain a vertical cross section, the sample irradiation current is gradually lowered in steps, in other words, the beam diameter is gradually reduced in steps toward finishing the vertical surface. However, when the sample irradiation current of the focused ion beam is switched from one step to another, the beam irradiation position change with respect to the sample. Accordingly, the irradiation position of the focused ion beam needs to be corrected at each step.